Have your class write a poem about their weekend. It can be about one particular event or a general summary of the whole weekend. It will help your students to see that they can come up with poetry in just a few minutes; that it doesn't have to be so difficult. You may want to suggest rhyming every other line in a stanza, or just the second and fourth. Provide a few samples of whatever kind of poetry you ask them to write, or just let them choose anything. They will have fun and this activity will help to encourage spontaniety.